1. Field of the Invention
The present invention relates to a semiconductor laser drive device used as a laser optical device such as a printer, a copy machine, an optical disk device and so on, and more particularly to an improvement of a distortion of an optical output waveform in a pulse modulation.
2. Description of the Related Art
Conventionally, a laser pulse light beam is used for scanning a photosensitive material provided in a laser printer, and optically recording and reproducing information on a disc in an optical disk device. Recently, a semiconductor laser, which has advantages such as miniaturization, low cost and low consumption of electric power, is used for generating a laser light beam. The semiconductor laser is supplied with a drive current, in which an intensity is varied like a pulse, to output a pulse modulated laser light beam.
In the semiconductor laser, however, the higher the temperature, the lower the optical output of the semiconductor laser. To compensate such a temperature characteristic of the semiconductor laser, an automatic power control (APC) circuit is provided in which the optical output is monitored by a photoreceptor element to control a drive current in such a manner that the optical output becomes constant regardless of the temperature.
On the other hand, it is known that, when a pulse-shape drive current is supplied to the semiconductor laser, a phenomenon such as relaxation oscillations occurs, in which, when the pulse-shape drive current is turned ON and OFF, a time-lag occurs in the optical output or a damping oscillation is superimposed on the optical output waveform. Particularly, a rise time from an idle state, in which a laser beam is radiated to some extent, to an operation state, in which the laser beam is radiated at a predetermined optical output level, is different from a rise time from a suspension state, in which the laser beam is turned OFF, to the operation state, and thus, the pulse waveform of the optical output does not exactly correspond to the pulse waveform of the drive current. Further, especially in the case of a high speed pulse modulation, since the relaxation oscillations affect the whole range of the pulse width, the waveform of the optical output cannot be rectangular, and becomes inaccurate as a signal.
Therefore, in a semiconductor laser drive device, a bias current is always supplied to the semiconductor laser, so that the time-lag and the relaxation oscillation are reduced. The optical output of the semiconductor laser hardly changes in a spontaneous emission condition, in which the drive current is lower than a threshold value, and drastically changes in proportion to the drive current in a laser oscillation condition in which the drive current is over the threshold value. Taking into consideration the optical output characteristics described above, the bias current is set to a constant value close to the threshold value, and only when a laser beam is to be output, a current for the laser oscillation is added to the bias current to obtain the drive current, which is supplied to the semiconductor laser.
However, the threshold value is changed depending upon a change of the ambient temperature, and becomes accumulatively high as the temperature becomes high. Therefore, to prevent a laser beam radiation when the laser beam should be extinguished, the bias current should be set to a value lower than a minimum value of a range within which the threshold value can be changed. Because of this, when the temperature becomes high, an effect of the bias current is lowered.
Therefore, an object of the invention is to provide a semiconductor laser drive device in which a high-speed pulse drive of the semiconductor laser can be performed without distorting the optical output waveform.
According to the present invention, there is provided a semiconductor laser drive device comprising a stabilizing processor, a sample hold processor, a generation processor, a switching processor and a control processor.
The stabilizing processor controls a drive current, used for adjusting an output power of a semiconductor laser to a predetermined level, so that the output power is stabilized. The sample hold processor is operated in a sampling mode, in which a control current is sampled, and in a holding mode, in which the control current is held. The generation processor generates a constant current. The switching processor selectively switches ON a supply of the constant current to the semiconductor laser and switches OFF the supply. The control processor controls the switching processor in synchronization with an activation of the sampling mode, so that one of the drive current and a bias current, which is obtained based on the control current and the constant current, is supplied to the semiconductor laser.